1. Field of the Invention
The present invention relates to an image forming apparatus and a method of controlling the image forming apparatus, and more particularly to an electrophotographic printer with a density detecting mechanism for correcting the print density of an image.
2. Description of the Related Art
Conventional color image forming apparatus, including a color electrophotographic printer, include a plurality of so-called process units, each process unit including a photoconductor, charging means, exposing means, and developing means. A tandem color image forming apparatus includes four process units: black (K), yellow (Y), magenta (M), and cyan (C) process units aligned along a paper transport path. When paper advances through these process units, toner images of corresponding colors are transferred one over the other onto the paper in registration.
In the aforementioned image forming apparatus, the print density of images can vary due to changes in the charging characteristic of toner and changes in the sensitivity characteristic of the photoconductive drum over time, and changes in environmental temperature and humidity. For this reason, the print density is detected at predetermined timings, for example, when the image forming apparatus is turned on and after a predetermined number of pages has been printed.
Japanese Patent Publication No. 2006-91736 discloses one such printer with a print density detecting mechanism. A test image is printed on a transport belt that transports the print medium thereon. A density detecting means reads the test image. The physical characteristics of the image forming apparatus including developing voltage and exposure time of the print engines are adjusted in accordance with the thus read test image, thereby correcting the print density of printed images.
The transport belts used in conventional printers are endless belts formed by joining two ends of a belt into a loop. The reflection characteristic of the joined portion may differ from that of the other portions of the belt, causing uneven reflection compared to other areas of the belt. Therefore, conventional printers suffer from a problem in that the light reflected from the jointed portion of the belt causes poor density correction results.